TFTs are widely used on active matrix substrates for display devices such as liquid crystal display devices. Commonly, channel layers of TFTs are made of silicon-type materials such as polycrystal silicon and amorphous silicon. Semiconductor compounds are now developed as next-generation materials which are alternative to silicon-type materials because they are capable of enhancing the electric properties of TFTs.
Patent Documents 1 and 2, for example, disclose oxide semiconductors containing In, Ga, and Zn, as semiconductor materials used in channel layers of TFTs. FIGS. 1, 10, and 18 of Patent Document 1 each disclose a phase diagram showing a preferable composition of In, Ga, and Zn in the oxide semiconductor. Patent Document 3 discloses an amorphous oxide semiconductor containing at least one of In, Ga, Al, Fe, Sn, Mg, Ca, Si, and Ge, which has a resistivity of 108 Ω·m. Moreover, Patent Document 4 discloses an oxide semiconductor containing at least one of In, Zn, and Sn.
Patent Document 5 discloses a semiconductor thin film comprising an amorphous film containing zinc oxide and indium oxide, which has a carrier density of 10+17 cm−3 or lower, a Hall mobility of 2 cm2/V·sec or more, and an energy band gap of 2.4 eV or more. Its preferable composition disclosed in Patent Document 5 is Zn/(Zn+In)=0.51 to 0.80. Moreover, Patent Document 6 discloses a semiconductor device in which a channel layer is constituted by x(Ga2O3).y(In2O3).z(ZnO) that satisfies about 0.75≦x/y≦about 3.15 and about 0.55≦y/z≦about 1.70.    [Patent Document 1]    Japanese Kokai Publication No. 2007-281409 (JP-A 2007-281409)    [Patent Document 2]    Japanese Kokai Publication No. 2008-277326 (JP-A 2008-277326)    [Patent Document 3]    Japanese Kokai Publication No. 2008-235871 (JP-A 2008-235871)    [Patent Document 4]    Japanese Kokai Publication No. 2008-166716 (JP-A 2008-166716)    [Patent Document 5]    Japanese Kokai Publication No. 2007-142195 (JP-A 2007-142195)    [Patent Document 6]    US 2007/0252147 A1